package com.tf.pemporaryfood.util;

import org.springframework.beans.factory.annotation.Value;

/**
 * 雪花算法 生产唯一id
 */
public class SnowFlakeUtils {
    private static class SnowFlake{
        private static final SnowFlake SNOW_FLAKE = new SnowFlake();
        // 起始的时间戳
        private final long START_STMP = 1480166465631L;
        // 每一部分占用的位数，就三个
        private final long SEQUENCE_BIT = 12;// 序列号占用的位数
        private final long MACHINE_BIT = 5; // 机器标识占用的位数
        private final long DATACENTER_BIT = 5;// 数据中心占用的位数
        // 每一部分最大值
        private final long MAX_DATACENTER_NUM = -1L ^ (-1L << DATACENTER_BIT);
        private final long MAX_MACHINE_NUM = -1L ^ (-1L << MACHINE_BIT);
        private final long MAX_SEQUENCE = -1L ^ (-1L << SEQUENCE_BIT);
        // 每一部分向左的位移
        private final long MACHINE_LEFT = SEQUENCE_BIT;
        private final long DATACENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
        private final long TIMESTMP_LEFT = DATACENTER_LEFT + DATACENTER_BIT;
        @Value("${snowFlake.datacenterId}")
        private long datacenterId; // 数据中心
        @Value("${snowFlake.machineId}")
        private long machineId; // 机器标识
        private long sequence = 0L; // 序列号
        private long lastStmp = -1L;// 上一次时间戳

        private long getNextMill() {
            long mill = getNewstmp();
            while (mill <= lastStmp) {
                mill = getNewstmp();
            }
            return mill;
        }
        public SnowFlake() {
            if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0) {
                throw new IllegalArgumentException("datacenterId can't be greater than MAX_DATACENTER_NUM or less than 0");
            }
            if (machineId > MAX_MACHINE_NUM || machineId < 0) {
                throw new IllegalArgumentException("machineId can't be greater than MAX_MACHINE_NUM or less than 0");
            }
            this.datacenterId = datacenterId;
            this.machineId = machineId;
        }
        //产生下一个ID
        public synchronized long nextId() {
            long currStmp = getNewstmp();
            if (currStmp < lastStmp) {
                throw new RuntimeException("Clock moved backwards.  Refusing to generate id");
            }

            if (currStmp == lastStmp) {
                //if条件里表示当前调用和上一次调用落在了相同毫秒内，只能通过第三部分，序列号自增来判断为唯一，所以+1.
                sequence = (sequence + 1) & MAX_SEQUENCE;
                //同一毫秒的序列数已经达到最大，只能等待下一个毫秒
                if (sequence == 0L) {
                    currStmp = getNextMill();
                }
            } else {
                //不同毫秒内，序列号置为0
                //执行到这个分支的前提是currTimestamp > lastTimestamp，说明本次调用跟上次调用对比，已经不再同一个毫秒内了，这个时候序号可以重新回置0了。
                sequence = 0L;
            }

            lastStmp = currStmp;
            //就是用相对毫秒数、机器ID和自增序号拼接
            return (currStmp - START_STMP) << TIMESTMP_LEFT //时间戳部分
                    | datacenterId << DATACENTER_LEFT      //数据中心部分
                    | machineId << MACHINE_LEFT            //机器标识部分
                    | sequence;                            //序列号部分
        }



        private static long getNewstmp() {
            return System.currentTimeMillis();
        }
        }
    public static long nextId() {
        return SnowFlake.SNOW_FLAKE.nextId();
    }
}
